Conceptualizing the Fe0/H2O System: A Call for Collaboration to Mark the 30th Anniversary of the Fe0-Based Permeable Reactive Barrier Technology

Abstract

Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), also known as the remediation Fe0/H2O system, a consensus on electro-chemical contaminant reduction was established during the 1990s and still prevails. Arguments against the reductive transformation concept have been regarded for more than a decade as ‘science denial’. However, is it the prevailing concept that denies the science of aqueous iron corrosion? This article retraces the path taken by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of the arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass-transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) the volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as an electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are inter-related, as the Fe0 surface is permanently shielded by a non-conductive oxide scale acting as a diffusion barrier to dissolved species and a barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and calls for alternative theories.

Influence of Water Salinity on the Efficiency of Fe0-Based Systems for Water Treatment

“…In particular, one investigator using ZVI4 might conclude that Cl − ions have no effect on Fe 0 efficiency, while a colleague working under similar conditions using ZVI1 or ZVI2 would come to a different conclusion. As discussed by several authors, the diversity of contaminants and Fe 0 materials, as well as the lack of unified or standardized experimental procedures are the three most important factors hindering the progress of knowledge in the design of sustainable Fe 0 -based remediation systems [17,19,30,77,92]. It can be seen that for all the 4 Fe 0 materials tested, the dye discoloration decreases in salt water compared to tap water, confirming the observations in Section 3.4 and further validating the working hypothesis.…”

“…Metallic iron (Fe 0 ), as a reactive material, has been tested and used extensively within the water treatment industry over the last three decades [11][12][13][14][15][16][17][18][19][20][21][22][23]. The discovery of Fe 0based permeable reactive barriers (Fe 0 PRBs) in the early 1990s was the starting point for intensive research, the results of which have been documented in several thousand scientific publications [16,[24][25][26][27].…”

“…Later, Ebelle et al [42] showed that the experimental conditions of Matheson and Tratnyek [35] were not suitable for them to draw their main conclusions. A number of papers have shown that the view that Fe 0 is a reducing agent is based on results from inappropriate experimental conditions [13,17,30,33,[43][44][45]. For example, experimental conditions (e.g., shaking, stirring) that prevent the diffusive transport of contaminants near the Fe 0 surface give reproducible results in the laboratory but are not relevant for Fe 0 PRBs in the field [13,18,28,30,[45][46][47][48][49].…”

Effects of solid additives and inorganic ligands on the efficiency of metallic iron- based water treatment system

“…In particular, one investigator using ZVI4 might conclude that Clions have no effect on Fe 0 efficiency, while a colleague working under similar conditions using ZVI1 or ZVI2 would come to a different conclusion. As discussed by several authors, the diversity of contaminants and Fe 0 materials, as well as the lack of unified or standardized experimental procedures are the three most important factors hindering the progress of knowledge in the design of sustainable Fe 0 -based remediation systems (Cao et al 2022, Gheju and Balcu 2023, Cao et al 2021c, Naseri et al 2017.…”